How the weather is measured

In this chapter:

Climatologists study climate which is weather over a long period of time

Weather is measured so that accurate forecasts can be made

Different aspects of nature are able to tell us about different aspects of the weather

Wind, temperature, air pressure, rainfall and humidity can all be measured using a variety of equipment

Weather satellites were first launched in 1960 to increase accuracy in weather measurements

Determining weather

We can see what the weather is doing at any given time but how can we determine what the weather will do in the future? Meteorologists are scientists who study the weather. They work with special equipment that allows them to predict future weather conditions. The areas meteorologists study include:

moving air (wind);

heat in the air (temperature);

water vapour in the air that then falls back to earth (precipitation); and

air pressure and the amount of water vapour in the air (humidity).

Climatologists are scientists who study patterns in the weather over a longer period. They study the weather over long periods of time to find out the climate of a specific area.

There are many modern tools used to predict weather today. Most of these tools are used on the ground, others are attached to balloons, inside aircraft, on satellites or on ships or buoys that are floating in the sea. This range of places allows meteorologists to collect greater amounts of information and more accurate data to work with.

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Weather is measured and predicted so that more accurate weather forecasts can be made. Forecasts can be seen daily on television, heard on the radio and read about in newspapers and on the internet. To give an accurate forecast meteorologists make thousands of observations and gain information from their equipment.

How nature tells us about weather

Before the development of sophisticated tools for measuring weather, people looked to nature for signs that would answer their weather questions. Farmers in particular watched the sky and the behaviour of animals to help them determine the approaching weather. Many of the signs were based upon superstition, whilst others proved to be accurate.

The more accurate signs of nature include:

A red sky at night meaning the next day will be fine

African guinea fowl pair off before it is due to rain, as it is believed they can hear the rumble of thunder hundreds of kilometres away.

A white ring around the moon often indicates that rain will fall in the following days, as the ring is made from ice crystals in the clouds.

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Natural indicators which are less likely to accurately predict weather include:

Chirping grasshoppers indicate warm, dry weather.

Busy bees are a sign of clear weather.

Open flowers and pine cones also signify fine weather.

People in different countries also hold different beliefs about natural weather indicators. The Spanish, for example, believe that when a donkey sways its head it will rain soon. Germans believe that when a cat washes itself it will rain soon.

Measuring wind

When measuring air movements (wind) meteorologists try to determine the direction the wind is coming from and the speed at which it is travelling. There are different instruments that are used to measure wind. Anemometers are used to measure wind speeds. These instruments consist of three or four hollowed cups that spin when the wind hits them. The higher the wind speed the faster the cups will move. Today, anemometers are connected to computers so that air movements can be measured more accurately.

Weather vanes are common tools used for measuring the direction of the wind. Many homes have weather vanes on their roofs. Weather vanes have been used for thousands of years. They consist of a moveable arrow which spins when the wind hits it. The point of the arrow will then point in the direction that the wind is coming from.

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Why not make your own pinwheel to see the effects of wind? The animation will show you how.

Measuring temperature

Air temperature is one of the most commonly measured weather conditions.

Thermometers are used to measure the amount of heat that is in the atmosphere. Thermometers are hollowed tubes that have a glass bulb at the bottom. The bulb contains a liquid, either alcohol or mercury. When the heat in the air increases, the liquid in the bulb expands and rises up the tube. When the heat in the air decreases (cools) the liquid contracts and falls back down the tube. The tube is calibrated, which means it has a scale that can be used to measure heat accurately. The number near the top point of the liquid indicates the current temperature.

Thermometers have not changed significantly over time. They are common fixtures in many homes, inside and outside and help people make decisions about what they need to wear each day.

Measuring air pressure

To measure air pressure (refer to Topic 2, Chapter 2) meteorologist use an instrument called a barometer. Air pressure can determine different weather conditions. Low pressure often means that the weather is about to become wet with rain or even a storm. High pressure often indicates fine weather.

Liquid barometers

There are two types of barometers: liquid barometers and non-liquid barometers. Liquid barometers are the older style of barometer and look similar to a clear jug with a spout. A liquid sits inside the jug, approximately half filling the jug. As the liquid rises and falls the air pressure can be measured. Nineteenth century sailors often used this type of barometer. When the liquid rose up the spout they knew that a low pressure system was coming, which meant stormy weather was on the way. These types of barometers were often called storm glasses.

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Non-liquid barometers

Non-liquid barometers are more commonly used today to measure atmospheric pressure. These types of barometers are also known as aneroid barometers. An aneroid barometer looks similar to a clock. To measure the air pressure these barometers use a box that has had its air removed. As air pressure increases it pushes against the sides of the empty box, which activates the needle to move around the face of the barometer. When the air pressure is low the sides of the empty box will expand, therefore activating the needle to move in the opposite direction around the dial.

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Measuring rainfall

Measuring rainfall is a simpler process than measuring other weather conditions. To measure rainfall a rain gauge is used. A rain gauge is a container that has been calibrated (marked with a series of measurement markings). When rain falls it will fill the container. When it stops the amount of water that fell in that area can be accurately measured using the markings.

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Meteorologists also use radars to detect precipitation, especially rain. Radar instruments work by emitting radio waves into the atmosphere. These waves are looking for raindrops. When they find one they are deflected back to the radar instrument which then creates an image of rain in the area.

Measuring humidity

Humidity is measured using an instrument called a hygrometer. Traditional hygrometers are made using two thermometers (one dry, one wet). One is kept in water, whilst the other is kept dry. The difference between the two thermometers indicates the humidity in the air. High humidity can cause fog or clouds.

Older hygrometers used hair to determine high and low humidity. When there is a large amount of moisture in the air, human hair will stretch. When moisture is low in the atmosphere human hair will contract. Human hair was placed inside a hygrometer and as it stretched it moved a measuring indicator.

Satellites

There have been many advances in weather-measuring technology. One of the most dramatic contributions to meteorology came in 1960 when the world's first weather satellite was launched into space.

Satellites are able to gather a lot more information about the weather on Earth as they can view events from their position in space. Satellites are also able to send back measurements, images of clouds and other weather conditions (cyclones, hurricanes). Satellites are also sensitive to heat and light and can therefore obtain information about temperatures across the Earth. Satellites have made weather measurements and predictions a lot more accurate.

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Before satellites meteorologists used weather balloons. These balloons reached the upper layers of the atmosphere and collected information which was then transmitted back to weather stations using small radio transmitters.